The invention relates to the field of atomic force microscopy and atomic force microscopes. In particular, it is directed to atomic force microscopes comprising an integrated, Fabry-Perot-like interferometer readout.
This section is intended to provide a background or context to the invention disclosed below. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived, implemented or described. Therefore, unless otherwise explicitly indicated herein, what is described in this section is not prior art to the description in this application and is not admitted to be prior art by inclusion in this section.
Scanning probe microscopy (or SPM) techniques rely on scanning a probe, e.g., a sharp tip, in close proximity with a sample surface whilst controlling interactions between the probe and the surface. An image of the sample surface can thus be obtained, typically in a raster scan of the sample. In a raster scan the probe-surface interaction is recorded as a function of position and images can be produced as a two-dimensional grid of data points.
The lateral resolution achieved with SPM varies with the underlying technique: atomic resolution can be achieved in some cases. Use can be made of piezoelectric actuators to execute scanning motions with precision and accuracy. The two main types of SPM are the scanning tunneling microscopy (STM) and the atomic force microscopy (AFM). In the following, acronyms STM/AFM may refer to either the microscopy technique or to the microscope itself.
In particular, an AFM apparatus or system allows the topography of a sample to be modified or sensed by a probe tip arranged at one end of a cantilever. As the sample is scanned, interactions between the probe and the sample surface cause pivotal deflection of the cantilever. The topography of the sample may thus be determined by detecting the deflection of the probe. In addition, the surface topography may be modified by controlling the deflection of the cantilever or the physical properties of the probe.
The probe usually consists of a sharp tip, which has a nominal tip radius on the order of 10 nm. Changes in the tip-sample interaction are often monitored using an optical lever detection system, in which a laser beam is reflected off of the cantilever and onto a position-sensitive photodetector. When scanning, a particular operating parameter is maintained at a constant level, and images are generated through a feedback loop between the optical detection system and the piezoelectric scanners. For a scanning stylus atomic force microscope, the probe tip is scanned above a stationary sample, while in a scanning sample design, the sample is scanned below a fixed probe tip.
Applications of AFM and other types of SPM keep on growing; they notably include biological materials (e.g., for studying DNA structure), polymeric materials (e.g., for studying morphology, mechanical response, and thermal transitions), and semiconductors (e.g., for detecting defects). In particular, AFM systems can be utilized to evaluate the surface quality of products such as contact lenses, optical components and semiconductor wafers after various cleaning, etching, or other manufacturing processes.
Three main imaging modes are known which can be used to obtain topographic images. These are the so-called: (i) contact mode; (ii) non-contact mode; and (iii) intermittent contact or tapping mode.
In contact mode, the probe is dragged across the surface. A constant flexure, or bend, in the cantilever is maintained. As the topography of the surface changes, the relative position of the tip with respect to the sample must be moved to maintain this constant deflection. The topography of the surface can thus be mapped using a feedback mechanism; the motion of the z-scanner corresponds to the sample topography. To minimize the amount of applied force used to scan the surface, low spring constant (e.g., k<1 N/m) probes are used. However, significant deformation and damage of soft samples (e.g., biological and polymeric materials) may occur during contact mode imaging. The combination of the applied normal force, the lateral forces (resulting from dragging the probe tip across the sample), and the small contact areas involved result in contact stresses that can damage the sample, the tip, or both.
To overcome this limitation, contact mode imaging can be performed within a liquid environment, which lowers problems due to, e.g., surface contamination such that lower contact forces can be used. Yet, imaging samples in a liquid environment are not always possible or practical.
To reduce the damages inherent to contact mode, the cantilever can be oscillated near its first (or fundamental) bending mode resonance frequency (e.g., on the order of 100 kHz) as the probe is raster scanned above the surface. This can be done in non-contact mode or tapping mode.
In non-contact mode, both the tip-sample separation and the oscillation amplitude are on the order of 1 to 10 nm, such that the tip oscillates just above the surface, essentially imaging the surface of, e.g., the contaminants. The resonance frequency and amplitude of the oscillating probe decrease as the sample surface is approached due to long-range forces extending above the surface (e.g., van der Waals). Either a constant amplitude or constant resonance frequency can be maintained through a feedback loop with the scanner and, just as in contact mode, the motion of the scanner is used to generate the surface image. To reduce the tendency for the tip to be pulled down to the surface by attractive forces, the cantilever spring constant is normally much higher compared to contact mode cantilevers. The combination of weak forces affecting feedback and large spring constants causes the non-contact AFM signal to be small, which leads to unstable feedback and requires slower scan speeds than either contact mode or tapping mode. Also, the lateral resolution in non-contact mode is limited by the nature of the long-range attractive forces and is normally lower than that in either contact mode or tapping mode.
Tapping mode tends to be more applicable to general imaging in air, particularly for soft samples, as the resolution is similar to contact mode, whereas the forces applied to the sample are lower and less damaging. In fact, a main disadvantage of the tapping mode relative to contact mode is the slower scan speed.
In tapping mode, the cantilever oscillates close to its first (fundamental) bending mode resonance frequency, as in non-contact mode. However, the oscillation amplitude of the probe tip is much larger than for non-contact mode, often in the range of 20 nm to 200 nm, and the tip makes contact with the sample for a short duration in each oscillation cycle. As the tip approaches the sample, the tip-sample interactions alter the amplitude, resonance frequency, and phase angle of the oscillating cantilever. During scanning, the amplitude at the operating frequency is maintained at a constant level, called the set-point amplitude, by adjusting the relative position of the tip with respect to the sample. In general, the amplitude of oscillation during scanning is large enough such that the probe maintains enough energy for the tip to tap through and back out of the surface.
As said, one of the main disadvantages of scanned probes in general, and AFM in particular, is the relatively low scanning speed. In fact, there is a trade-off between scanning speed and wear reduction. In general, methods that reduce sample damage tend to slow down the scanning process. These methods mostly rely on avoiding sliding friction, using dynamic techniques, such as the tapping or non-contact modes described above.
On the whole, however, AFM technology is limited by the slow response time of the feedback scheme used to guide the tip across the surface topography. The response time is impacted by the limited mechanical response times of the stage moving the tip (or the sample), the averaging techniques used in the dynamic mode to interpret the received signals and the limited sensitivity at high bandwidth of the detection schemes.